1. Field of the Invention
The present invention relates to means for coating semiconductive materials with electrostatically assisted coating apparatus, in general, and to such apparatus for coating a moving web of such materials, in particular.
2. Description of the Prior Art
In the manufacture of various coated products it is often essential that coating materials applied to such products be of uniform thickness. In, for example, the continuous manufacture of coated photographic sheet materials, a non-uniform thickness coating applied to a moving web of said material will require considerably more drying time for drying the thicker portions of a nonuniform coating than will be required for drying the thinner portions of said nonuniform coating. In addition, a temperature gradient that is optimum for drying said thicker coating portions is often excessive for optimum drying of said thinner coating portions. Drying time is usually the major factor limiting maximum production rates of many coated products. Also, many properties of photographic film, for example, such as sensitivity to light, color saturation, etc., can be adversely affected when constructed with nonuniformly coated sheet materials.
Mechanical devices generally employed in the web coating art, such as doctor blades, scrapers and the like, have controlled the uniformity of web coating thickness to a limited degree. However, in the production of photographic film, for example, such contact devices have a propensity for inducing surface defects in the film coatings and in addition, these contact devices very often have a detrimental effect on the sensitometry of a finished photographic film product.
One of the most effective coating thickness control apparatus in present day use in the coating industry utilizes electrostatics to uniformly deposit coating materials on products to be coated. In the production of photographic film, for example, a web or sheet of material to be coated is passed between an electrically conductive support or backing roller and a coating applicator from which coating material flows onto a surface of said web. An electrostatic field is established across the gap between the coating applicator and the backing roller by a high voltage power supply whose output terminals are normally connected between said applicator and said roller. The electrostatic field causes a coating, of uniform thickness, to be deposited on the web surface to be coated, and permits larger coating gaps to be employed between said coating applicator and the material to be coated. While the voltage magnitude established between said applicator and said roller is less than that required to generate corona, said magnitude often exceeds 3KV DC.
The use of electrostatically assisted coating apparatus employing voltages in the vicinity of 3KV or more is relatively effective when coating dielectric materials or materials that have a relatively high electrical resistance. However, if such apparatus is employed to coat semiconductive materials, excessive heat-generating current levels could result because of the lower electrical resistance of such materials, and this excessive heat would have a detrimental effect on the quality of such materials. The greater the conductivity of the semiconductive materials, the greater the magnitude of harmful heat-producing current that would be generated for any given level of electrostatic assist.